I bet 25 grains of that mixture for 55 grain 223 will plink decently.
I really don't think mixing powders of similar burn rates, for the same caliber, with comparable grain ranges, really matters that much. If barely at all. Function wise. Its just not going to be that accurate, because the ES is going to be larger than you are used to due to the inconsistency of one to the other.
its pure luck that bechmark and H335 are almost the same exact burn rate.
I really dont think this is a big deal, I would shoot it. And I would use around 24-25 grains for 55 grain 223 2.255 OAL.